Permit body-text assertions on both the text part and the html part

[yfeldblum]

Permit body-text assertions on both the text part and the html part.

let(:message) { SomeMailer.some_message("some-arg") }

it "has the expected text in the text part" do
  message.should have_text_body_text "*asdf*"
end

it "has the expected text in the html part" do
  message.should have_html_body_text "<b>asdf</b>"
end